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Evidence of earlier high altitude sea rise discovered by scientists



  Fertile overgrowth

A close-up of the bulbous stalactitic element of a frantic growth on speleothems (POS). Credit: University of New Mexico

Researchers discover evidence of previous sea level rise at current atmospheric carbon dioxide levels. The cave in Mallorcan provides 4 million years old geological evidence that provides new insights on the global sea level .

An international team of scientists studying evidence preserved in speleothems in a coastal cave illustrate that more than three million years ago – a time when the earth was two to three degrees Celsius warmer than the pre-industrial era – the sea level was as much as 16 meters higher than today. Their findings represent significant consequences for understanding and predicting the current sea level rise in the midst of a warming climate.

The researchers, including Professor Yemane Asmerom and sr. Research Scientist Victor Polyak of the University of New Mexico, University of South Florida, Universitat de les Illes Balears and Columbia University published their findings in today's issue of the journal Nature . The analysis of deposits from Artà Cave on the island of Majorca in the western Mediterranean produced sea levels that serve as a target for future studies of ice sheet stability, calibration of ice sheet models and forecasts for future sea level rise, the researchers say.

The sea level rises as a result of melting ice sheets, such as those covering Greenland and Antarctica. However, how much and how fast the sea level will rise during the warm-up is a question that the researchers have worked to answer. Reconstructing changes in ice sheets and sea level during earlier periods when the climate was naturally warmer than today, provides an earth-scaled laboratory experiment to study this issue according to USF Ph.D. student Oana Dumitru, the lead author, who did much of her dating work at UNM under the leadership of Asmerom and Polyak.

"Limiting sea level rise models due to increased warming depends critically on actual sea level measurements," Sa Polyak said. "This study provides very robust sea level measurements during the Pliocene."

"We can use knowledge gained from previous warm periods to set ice sheet models that are then used to predict future ice sheet responses to current global warming," said USF Institute of Earth Sciences Professor Bogdan Onac.

The project focused on The deposits are formed in coastal caves at the interface between brackish water and cave air each time the old caves were flooded by rising sea levels. In Artà Cave, located within 100 meters of the coast, the water table – and was previously – coincides with the sea level , says Professor Joan J. Fornós, Universitat de les Illes Balears.

The researchers discovered, analyzed and interpreted six of the geological formations found at altitudes 22.5 to 32 meters above today's sea level. Careful sampling and laboratory analyzes of 70 samples resulted in ages that ranged from 4.4 to 3.3 million years old BP (before its presence), indicating that the cave deposits were formed during the Pliocene era. The ages were determined using uranium-lead radiometric dating in UNM's radiogenic isotope laboratory.

  Phreatic overgrowth on Speleothems

The bulbous stalactitic element in the center of this photo is a frantic growth of speleothems (POS) that grew exactly to sea level and therefore represents a correct relative sea level ~ 3.5 million years ago. Credit: University of New Mexico

“This was a unique convergence between an ideal natural environment developed by the team of cave scientists and the technological advancements we have achieved over the years in our lab at the University of New Mexico, Sa Asmerom. "Sensible investments in instrumentation and techniques result in such high dividends."

"Sea-level changes at Artà Cave can be caused by melting and growing of ice sheets or by raising or lowering the island itself," said Columbia University Assistant Professor Jacky Austermann, a member of the research team. She used numerical and statistical models to carefully analyze how much uplifting or lowering could have happened since the Pliocene, subtracting this from the height of the formations they examined.

An important range of particular interest during the Pliocene is the middle of the Piacenzian Warm Period – some 3,264 to 3,025 million years ago – when the temperature was 2 to 3 ° C higher than pre-industrial levels. "The interval also marks the last time the Earth's atmospheric CO 2 was as high as it is today, providing important clues as to what the future holds for the current anthropogenic warming," says Onac.

This study found that during this period the global mean height was as high as 16.2 meters (with an uncertainty range of 5.6 to 19.2 meters) above the present. This means that even if atmospheric CO 2 is stabilized at current levels, the global mean sea level would probably rise at least as high, if not higher, scientists conclude. In fact, it is likely to rise higher due to the increase in ocean volume due to rising temperature.

"Given today's melting pattern, this extent of sea-level rise would probably be caused by a collapse of both Greenland and West Antarctic ice sheets," Dumitru said.

The authors also measured the sea level at 23.5 meters higher than presently about four million years ago during the Pliocene Climatic Optimum, when global average temperatures were up to 4 ° C higher than pre-industrial levels. "This is a possible scenario unless active and aggressive reduction of greenhouse gases in the atmosphere is not implemented," Asmerom said.

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The research team included research students and lead authors Oana A. Dumitru and Professor Bogdan Onac, both at the School of Earth Sciences, University of South Florida, Professor Jacqueline Austermann, Department of Earth and Environmental Sciences, Columbia University, Senior Research Scientist Victor Polyak and Professor Yemane Asmerom at the Department of Earth and Flat Sciences, University of New Mexico at Albuquerque, and Professor Joan Fornós, Associate Professor Joaquín Ginés and Angel Ginés at the Universitat de les Illes Balears in Mallorca.

This research is the result of a National Science Foundation (NSF) collaboration project between the University of South Florida and the University of New Mexico and part of the USF-UIB bilateral agreement, and has been funded by the NSF and the Spaniards of the State Research Agency.

Reference: "Limitations of the Global Mean Height in Pliocene Heat" by Oana A. Dumitru, Jacqueline Austermann, Victor J. Polyak, Joan J. Fornós, Yemane Asmerom, Joaquín Ginés, Angel Ginés, and Bogdan P. Onac, August 30, 2019 , Nature .

DOI: 10.1038 / s41586-019-1543-2


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